Aside from their different 3D technologies, the major appreciable difference between the two has nothing to do with the picture: the X900A has bulky built-in speakers on either side of the screen, where the X850A is a more traditional design.

Otherwise, the X850A and X900A seem pretty much identical judging from their specifications. UHD/4K in these TVs' (and most others') case means a pixel count of 3,840x2,160 -- four times as many pixels as today's 1080p TVs. The advantage, according to its proponents, is an even sharper picture. One problem, according to us, is that you'll have to sit very close, especially to a screen this small, to appreciate the difference (see our in-depth look at the 84-inch Sony for more). There are many other issues, too, to the extent that we currently consider 4K TVs pretty stupid.

To its credit, Sony is starting to address one big problem, lack of content, by selling a 4K video player. Today the company announced that the video download service for that player is now live, offering more than 70 feature films and TV shows. I also applaud Sony's decision to buck the "UHD" moniker in favor of the more established, and frankly catchier, "4K."

Aside from 4K, the XBR-X850A is packed with picture-quality enhancements, although unlike the 2012 XBR-HX950, it doesn't have full-array local dimming. On the other hand, its edge-lit display does include edge-lit local dimming (more info), which in Sony's implementation works better than just about any other example of its kind, judging from models like the W900A we liked so much.

The X850A also has what Sony calls "Triluminos Display LED backlighting." No, it's not the same as the three-color LED backlight of the vaunted XBR8 from 2008. Instead it delivers somewhat improved color response, judging again from our W900A review. Check out our in-depth look at the technology for more nuts and bolts.

The XBR-X850A also offers all of the features of the step-down sets like the R550A, including NFC and RVU (click through for an explanation). Beyond those fancy extras is a full helping of Smart TV, including the Sony Entertainment Network.

About The Author

Section Editor David Katzmaier has reviewed TVs at CNET since 2002. He is an ISF certified, NIST trained calibrator and developed CNET's TV test procedure himself. Previously David wrote reviews and features for Sound & Vision magazine and eTown.com. He is known to two people on Twitter as "The Cormac McCarthy of consumer electronics."